This invention relates to strain indicators of the type shown in U.S. Pat. Nos. 3,602,186 and 3,799,108. More particularly, this invention relates to a strain indicator apparatus of the type wherein a change in length of a strain member produces a change in color of a visual indicator.
In many assembly operations where fastening devices are used it is desirable to inspect the assembled fastener to determine that it has been taken up an appropriate amount. The fastener may be, for example, a stud, a bolt, a rivet or the like.
Several prior art techniques have been utilized for determining when a fastener has been properly taken up. One such technique involves the use of a torque wrench in which a specified number of foot pounds may be verified as having been applied to a fastener in the assembly thereof. Other prior art techniques have made use of the fact that take-up of a fastener will set up certain stress levels in the fastener which may be measured as, for instance, by means of stress sensitive material. Still further prior art techniques have made use of the fact that take-up of a fastener produces an elongation of the fastener which may be measured utilizing well known techniques such as direct measuring devices.
The above examples demonstrate that fastener take-up may be measured by determining either torque, stress or strain values in a fastener.
Since torque, stress and strain are all interrelated, it is possible to determine the stress levels in a fastener knowing the strain of the fastener. Similarly, it is possible to determine torque values knowing the stress imparted to the fastener at any point. Thus, for purposes of illustration, the terms torque, stress and strain may be used interchangeably. The device of this invention is termed a strain indicator because the elongation of the fastener is being sensed in order to determine a fully made-up condition of the fastener. The term strain indicator should not be considered as limiting since in its broadest sense the device of this invention may be considered a stress or torque indicator as well. In U.S. Pat. Nos. 3,602,186 and 3,799,108 there are shown and described strain indicators of the type wherein a change in length of the fastener causes an indicator area of a pin to separate from a window with the result that light-absorbing indicator fluid is caused to flow between the indicator area and the window.
This invention is directed to an improved strain indicator of the type wherein a change in length of the fastener produces a visual indication at the head of the fastener.